Various functions related to prime factorisation.
Many of these functions use the prime factorisation of an Integer.
If several of them are used on the same Integer, it would be inefficient
to recalculate the factorisation, hence there are also functions working
on the canonical factorisation, these require that the number be positive
and in the case of the Carmichael function that the list of prime factors
with their multiplicities is ascending.

The implementation is not very optimised, so it is not suitable for factorising numbers
with several huge prime divisors. However, factors of 20-25 digits are normally found in
acceptable time. The time taken depends, however, strongly on how lucky the curve-picking
is. With luck, even large factors can be found in seconds; on the other hand, finding small
factors (about 12-15 digits) can take minutes when the curve-picking is bad.

Given enough time, the algorithm should be able to factor numbers of 100-120 digits, but it
is best suited for numbers of up to 50-60 digits.

factorise n produces the prime factorisation of n, including
a factor of (-1) if n < 0. factorise 0 is an error and the
factorisation of 1 is empty. Uses a StdGen produced in an arbitrary
manner from the bit-pattern of n.

defaultStdGenFactorisation first strips off all small prime factors and then,
if the factorisation is not complete, proceeds to curve factorisation.
For negative numbers, a factor of -1 is included, the factorisation of 1
is empty. Since 0 has no prime factorisation, a zero argument causes
an error.

stepFactorisation is like factorise', except that it doesn't use a
pseudo random generator but steps through the curves in order.
This strategy turns out to be surprisingly fast, on average it doesn't
seem to be slower than the StdGen based variant.

Factor sieves

factorSieve n creates a store of smallest prime factors of the numbers not exceeding n.
If you need to factorise many smallish numbers, this can give a big speedup since it avoids
many superfluous divisions. However, a too large sieve leads to a slowdown due to cache misses.
The prime factors are stored as Word16 for compactness, so n must be
smaller than 2^32.

sieveFactor fs n finds the prime factorisation of n using the FactorSievefs.
For negative n, a factor of -1 is included with multiplicity 1.
After stripping any present factors 2, the remaining cofactor c (if larger
than 1) is factorised with fs. This is most efficient of course if c does not
exceed the bound with which fs was constructed. If it does, trial division is performed
until either the cofactor falls below the bound or the sieve is exhausted. In the latter
case, the elliptic curve method is used to finish the factorisation.

Trial division

trialDivisionTo bound n produces a factorisation of n using the
primes = bound@. If @n@ has prime divisors @ bound, the last entry
in the list is the product of all these. If n <= bound^2, this is a
full factorisation, but very slow if n has large prime divisors.

Partial factorisation

smallFactors bound n finds all prime divisors of n > 1 up to bound by trial division and returns the
list of these together with their multiplicities, and a possible remaining factor which may be composite.

A wrapper around curveFactorisation providing a few default arguments.
The primality test is bailliePSW, the prng function - naturally -
randomR. This function also requires small prime factors to have been
stripped before.

curveFactorisation is the driver for the factorisation. Its performance (and success)
can be influenced by passing appropriate arguments. If you know that n has no prime divisors
below b, any divisor found less than b*b must be prime, thus giving Just (b*b) as the
first argument allows skipping the comparatively expensive primality test for those.
If n is such that all prime divisors must have a specific easy to test for structure, a
custom primality test can improve the performance (normally, it will make very little
difference, since n has not many divisors, and many curves have to be tried to find one).
More influence has the pseudo random generator (a function prng with 6 <= fst (prng k s) <= k-2
and an initial state for the PRNG) used to generate the curves to try. A lucky choice here can
make a huge difference. So, if the default takes too long, try another one; or you can improve your
chances for a quick result by running several instances in parallel.

curveFactorisation requires that small prime factors have been stripped before. Also, it is
unlikely to succeed if n has more than one (really) large prime factor.

Single curve worker

montgomeryFactorisation n b1 b2 s tries to find a factor of n using the
curve and point determined by the seed s (6 <= s < n-1), multiplying the
point by the least common multiple of all numbers <= b1 and all primes
between b1 and b2. The idea is that there's a good chance that the order
of the point in the curve over one prime factor divides the multiplier, but the
order over another factor doesn't, if b1 and b2 are appropriately chosen.
If they are too small, none of the orders will probably divide the multiplier,
if they are too large, all probably will, so they should be chosen to fit
the expected size of the smallest factor.

sieveTotient ts n finds the totient π(n), i.e. the number of integers k with
1 <= k <= n and gcd n k == 1, in other words, the order of the group of units
in ℤ/(n), using the TotientSievets.
First, factors of 2, 3 and 5 are handled individually, if the remaining
cofactor of n is within the sieve range, its totient is looked up, otherwise
the calculation falls back on factorisation, first by trial division and
if necessary, elliptic curves.

sieveCarmichael cs n finds the value of λ(n) (or ψ(n)), the smallest positive
integer e such that for all a with gcd a n == 1 the congruence a^e ≡ 1 (mod n) holds,
in other words, the (smallest) exponent of the group of units in ℤ/(n).
The strategy is analogous to sieveTotient.